Engineering Post Frame Buildings
Engineering Post Frame Buildings
(OP)
I'm getting ready to do an analysis of a post frame building and I am still hunting down every resource I can find to educate myself since this is the first post frame building I have ever done. I've managed to acquire a copy of the 1999 Post-Frame Building Design Manual published by the NFBA, which appears to be the defacto standard for pole building engineering. However, online I have found other misc. papers describing a "simplified" method for designing post frame buildings.
I am wondering what others typically use as their reference and what are your thoughts on the simplified methods (Don Bender and Drew P. Mill).
I've also just noticed that the second edition of the NFBA manual has now come available.
http://www.nfba.org/index.html
Has anyone had a chance to purchase it and compare it with the 1999 edition (First Edition)?
I am wondering what others typically use as their reference and what are your thoughts on the simplified methods (Don Bender and Drew P. Mill).
I've also just noticed that the second edition of the NFBA manual has now come available.
http://www.nfba.org/index.html
Has anyone had a chance to purchase it and compare it with the 1999 edition (First Edition)?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com






RE: Engineering Post Frame Buildings
I suppose you could combine the two also. What do you have?
Mike McCann, PE, SE (WA)
RE: Engineering Post Frame Buildings
I also use:
- ANSI/ASAE EP 559 - Mechanically Laminated Columns
- ANSI/ASAE EP486.1 - Shallow Post Foundation Design
- Research papaer FPL-RP-528 by the United States Department of Agriculture
Might I also recommend if you are really stuck, Try contacting Dr. Kris Dyck at the University of Manitoba (you can get his info from their staff directory page). He is the Pole shed and hay bale house specialist.RE: Engineering Post Frame Buildings
Is there any specialized software out there already, my ignorance may have me reinventing the wheel here.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
At least not that I've found. To be completely honest you are in for a battle with the contractor again.
These will not calc out the way he's expecting. You are going to need to use your 'What's reasonable?' hat a lot when designing these.
You can basically throw deflection requirements out the window and be prepared to take every member right to it's limit.
How tall? Dirt Floor? 4 foot embedment is unlikely to work unless the posts are spaced tightly.
RE: Engineering Post Frame Buildings
It's only a 20x30 with posts at 10' on center, so four posts per side. A 5-1/8 x 10-1/2 Glulam beam spanning the posts on each side with mfg. trusses at 24" o/c.
14' eave height, 4/12 pitch, 24" overhangs all the way around. Originally an open design but the owner has framed in walls between the posts, so its now an enclosed structure.
Post depth is 4' with 18" Dia. holes, back filled with concrete to grade. The original plans called out #4 rebar drilled through the posts for uplift.
This is a case of the structure being built without a permit and now the building dept. has required it be engineered. The owner had assumed that he did not need a permit since the square footage was under 800 sqft.
The addition of framing and sheathing to the walls will add to the lateral strength of the structure however I think I will first neglect that contribution and see if it works with just the capacity of the poles and roof diaphragm.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
Make sure to use low importance when determining your loading.
Good luck with this, you're in for another tough one.
RE: Engineering Post Frame Buildings
I am trying to determine whether I should analyze the structure with or without diaphragm action. I am looking for pros and cons of each method and what others typically do for simple post frame structures.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
A true post-frame structure would have let-in diagonal braces framing from the columns to the beams above, columns not extending into the foundation, and hence pinned at the footing.
Mike McCann, PE, SE (WA)
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
Mike McCann, PE, SE (WA)
RE: Engineering Post Frame Buildings
A post would be considered constrained at the ground surface if a concrete slab or other means was present. A post with a full collar acts just like a post without a collar, except it is much wider, therefore reducing the lateral stress on the soil. Again EP486 will answer all these questions.
A true pole structure will likely need knee and wye braces at the top of the posts, otherwise the posts act as a cantilevers. Fixed at about d/3 below the ground and pinned at the top. The Ke factor of 2 for this condition increases post sizes drastically. This may work with smaller structures. My experience is mostly 40-50' wide structures with peak heights of 20' or so. These structures typically require 8x8 or 8x10 SYP posts. These structures are designed for 60 psf gsl.
RE: Engineering Post Frame Buildings
From a pole frame to a shearwall structure. I used to make a pretty good living off of contractors who did that.
RE: Engineering Post Frame Buildings
In the NFBA they have tables for diaphragm and shear wall assemblies that they tested just using the metal decking as the sheathing for the walls and roof.
In my experience designing pole barns is a pain. The owner always thinks it should be cheaper.
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
I've been doing a lot of reading the last couple of nights (last night up until 4:00AM, not sure how productive that was). DAFI is used to analyze the structure if your assuming diaphragm action, which I will probably want to do in this case since I can get some of the transverse load off of the post frames and into the gable end shearwalls.
Unlike conventional framing there are not holdowns but I'm assuming that the embedded posts at the corners of the structure can serve the same purpose.
My other burning question right now it how to deal with the tributary area for lateral loads due to wind. With conventional stud walls and roofs half the wall height and the roof height is tributary to the top of the walls and the shear load is applied at the top of the walls. For a standard post frame structure using girts the load distribution would seem to be different, wouldn't all of the wall area be considered for lateral loading to the posts in addition of course to the lateral loads coming from the roof.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
The NFBA manual goes through that as well.
RE: Engineering Post Frame Buildings
If you are designing the posts to cantilever then your thoughts are correct, when designing the posts you need to account for the entire wall, and the roof contribution.
If you're trying to dump some of the lateral shear load into the end walls the hardest part is determining how much makes it there as opposed to how much the columns end up taking.
RE: Engineering Post Frame Buildings
must equal at each column location.
Simpler to design the columns for their tributary area in the short direction and the long sidewall shearwall for half the endbay area force.
Mike McCann, PE, SE (WA)
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
Mike McCann, PE, SE (WA)
RE: Engineering Post Frame Buildings
If you put 6" of gravel in the bottom of the hole I'm assuming that you use some type of collar (wood or concrete) otherwise the vertical bearing pressure of the posts would punch right down through that gravel. Everything I've seen so far usually calls out at least 6" of concrete in the bottom of the hole as a footing for the posts. The NFBA manual states that any friction between the posts and soil cannot be considered for vertical bearing.
With regards to the lateral load applied to the cantilevered posts: Do I mass all of the load from the roof and walls (trib. load by area for that post, transverse direction) into a point load at the eave height?
or
Do I apply a point load for the roof load at the eave height and then a rectangular distributed load from the walls to the full height of the post from the eave to grade?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
Most shearwall calcs don't work as the posts are spaced too far apart for the plywood to act properly.
And the metal diaphragms don't have many published values and the ones I've seen lead me to assume they're only good for 100plf and don't screw around with the fastening (pun intended).
All of the ones I've seen designed are cantilevered columns.
RE: Engineering Post Frame Buildings
I guess with this particular case I will have to assume that the endwall shear is simply taken up by the two endwall posts, cantilevered.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
I then played with the spring constant until the diaphragm shear was within my allowable.
It never ends up being worth the fees but that's how I had to do it.
RE: Engineering Post Frame Buildings
http://design.medeek.com/resources/postframe/2015-...
As you can see I'm still establishing the basic loads and deflections, however in a few hours I should be able to determine if this particular structure will actually "work". The kind of cool thing with this particular calculator is that I've integrated the wind load calculator into a separate sheet so changing wind loads is a "breeze".
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
Current Spreadsheet and PDF output is here:
http://design.medeek.com/resources/postframe/
The ultimate wind speed is 155 mph (120 mph ASD) or 100 mph fastest mile. I've taken a reduction of .87 in the wind speed in order to bring this down to a Risk Category I building, which drops my ultimate wind speed to 135 mph.
Even so the combined stress on the 6x6 HF No. 2 central posts (windward side) are at 151% of the allowable.
What else more can I do to make this building work?
I've gone through the spreadsheet fairly carefully looking for any calculations errors or flaws in the methodology but it appears to be mostly sound.
Granted I haven't checked uplift yet and C&C out of plane loads to the roof and wall sheathing but that will probably be fine based on previous experience.
I've also taken a vertical pressure increase per ASAE486.1 for the footings since the jurisdiction has not adopted the more recent codes and their code books still reference this document and not the more recent ASAE486.2.
The really big problem I have if I can't get the numbers to work is that I probably will not get paid for this job. The client does not want to hear that his RV garage has to be torn down because my numbers don't work. Even if one was to retrofit the central posts by sistering 2x6's onto the appropriate face, this would still not really help with the max. moment occurring right at the ground line. If I can't get it to work the customer will probably hire another engineer who can make it work.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
Have you really trimmed down your roof dead loads (I know it hurts the uplift calcs but those are easier to deal with usually). It likely won't get you to passing but it may trim a few percent off of the code check.
I would get confirmation on the post species and grade if possible that way you have an accurate capacity.
If you got this down to 120% or less I would be impressed. The existing ones don't calc out as we all have been saying the whole time.
You could knee brace the columns and it might get you enough.
RE: Engineering Post Frame Buildings
Now the final piece of the puzzle...
I'm looking at the uplift resistance of a 18" dia. by 4' deep post hole backfilled with concrete. At 150 pcf the concrete donut basically weighs 934 lbs. My basic uplift from the wind combined with 0.6 of the dead loads is only 703 lbs so that works.
However, to get an additional margin I would like to include a value for the skin friction between the soil and the concrete cylinder. I'm not seeing an example of that calculation in any of the resources and texts I'm looking at.
Then of course there is the problem of the corner posts which are acting as the shearwall chords and resisting the lateral loads. They have a certain uplift which is countered by the roof and wall dead load to some extent 7.) 0.6D + 0.6W and the uplift capacity of the backfill.
In my opinion these types of structures are an engineer's nightmare. There is way too much going on with these posts both vertically and laterally. A conventionally framed structure distributes the loads far better and is easier to analyze. I've spent the entire week trying to spreadsheet this thing and I know my analysis is still far from adequate or complete.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
If I assume a 6" deep circular collar at the base of the post (4' embedment) I then am able to take the cone of soil per 8.9.4 of the NFBA manual which is 3,893 lbs.
However, if I assume concrete backfill to the surface then all I have is the weight of the concrete in the hole minus the post area which gives 934 lbs.
Does this not seem somehow counter intuitive? Wouldn't the concrete lift some of the soil with it in a full on uplift situation?
The question is how to quantify this.
8.9.3 of the NFBA manual states that concrete backfill gives an uplift resistance of the mass of the concrete plus skin friction but then gives no further guidance.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
I'm thinking it will put a concentrated load on the corner post 4' feet down from the eave as well as a similar load on the 20' fink truss above. There is a double 2x6 top plate below the fink truss but this is not the same as a deep beam that can accommodate the bending loads.
Has anyone run into something like this before? Any suggestions?
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
EIT
www.HowToEngineer.com
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
It's Friday...
Mike McCann, PE, SE (WA)
RE: Engineering Post Frame Buildings
...and a 'good' Friday for some...
RE: Engineering Post Frame Buildings
The full analysis of the post frame is here for those interested:
http://design.medeek.com/resources/postframe/
Assuming that the structure transfers the bulk of its lateral loads to the end walls (shearwalls A and B) and the post frames on grid lines 2 and 3 do not offer any significant resistance I get a shear load of Vw = 1630 lbs, which doesn't sound like a lot.
So I then run the following manual calcs to determine if a 4' diagonal knee brace will work on the one end wall:
The axial force in my knee brace is a whopping 4,034 lbs.
I then check to see what effect that might have on one of my 6x6 HF No. 1 corner posts:
As you can see it does not work, not even close.
The loads on the horizontal header would also require some beefing up of the gable end truss bottom chord but that can easily be arranged.
With respect to the knee brace and the corner post I don't think sheathing the knee brace will do much to decrease the bending moment in the corner post but I could be wrong. Any suggestions?
I thought of a few different solutions:
1.) Tell the client knee bracing won't work and make him install two additional end wall posts to create a 12' wide door as I originally envisioned he might do.
2.) Sister another 6x6 or even 4x6 post to the existing corner post to get more bending strength, attached the to posts with some counter sunk lag bolts and sheathing.
3.) Somehow analyze the knee brace differently so that my axial force in this member is decreased?
4.) Re-analyze the entire post frame building treating the roof diaphgram as rigid, essentially a three wall structure. The problem with this is that it doesn't really agree well with other accepted methods of assigning shear loads to post frame buildings (ie. NFBA manual etc...)
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
Actually, before I ran with RISA, I first checked to see what would happen if just the embedded posts tried to resist the load without any knee braces and assuming 815 lbs at 14 ft off the ground I get a 11,410 ft-lbs ground line moment. That certainly wasn't going to work.
Anyhow, even with the reduction in moments and assuming a DF No. 1 post the numbers are still too high. Bending wise it comes in at about 98% but then if you add in the axial and calculate the combined stress index its about 170%.
However, putting it into RISA was a good idea since I got a deflection at the eave that manually I could never have reasonably calculated. The deflection is about 2.2", which is extremely large compared to the deflection of the roof diaphragm (7/16" OSB sheathing on 24" o/c trusses). So given that I can assume a rigid diaphragm and basically throw most of my calculations out the window...
The question is now do I really need any sort of knee bracing at all. The three other walls are fully sheathed with 15/32 plywood, plenty of shearwall and stiffness and no windows or doors.
I kind of wish I knew RISA a little better, I would probably put the whole structure into it and let it figure out what component (post, shearwall, knee braces etc...) is actually taking the lateral loads.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
I think I finally have a handle on this one but after so much trouble in making it "work" I'm left thinking that maybe there is something else that I have not accounted for that will pop up and invalidate the entire updated set of calculations.
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
EIT
www.HowToEngineer.com
RE: Engineering Post Frame Buildings
There was a fair amount of discussion a while back as to whether 3-sided shear wall structures were acceptable. I'll continue to watch this thread with interest.
RE: Engineering Post Frame Buildings
RE: Engineering Post Frame Buildings
A confused student is a good student.
Nathaniel P. Wilkerson, PE
www.medeek.com
RE: Engineering Post Frame Buildings
EIT
www.HowToEngineer.com